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What did Julius Elster and Hans Geitel learn from their early two-electrode device?

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thermionic power converter Schematic of a basic thermionic converter.

Between 1882 and 1889, Julius Elster and Hans Geitel developed a sealed device featuring two electrodes, one of which could be heated while the other remained cool. Through their experiments, they learned the following regarding the flow of electric current:1

At fairly low temperatures, electric current flows with little resistance if the hot electrode is positively charged.1
At moderately higher temperatures, current flows readily in either direction.1
At even higher temperatures, electric charges from the negative electrode flow with the greatest ease.1

These observations contributed to the early understanding of thermionic emission, the process by which heat causes the discharge of electrons from materials.21